In recent years, various oligonucleotides such as DNAs and RNAs have come to be used for treatment, diagnosis, etc. For example, specific gene knockdown techniques using RNA interference (RNAi) have attracted attention as nucleic-acid technologies. RNAi is a phenomenon in which the function of a gene is inhibited by the action of a double-stranded RNA (dsRNA) having a sequence homologous to the gene. Nucleic acid medicines using this RNAi are highly expected as next-generation therapeutic medicines.
On the other hand, studies have also been made to chemically modify oligonucleotides to impart new functions that natural forms do not have. The present inventors have also developed a technique for introducing an ethynyl group at the end of an oligonucleotide and further modifying the ethynyl group with a new substituent, such as a benzene ring, by utilizing a click reaction (Patent Literature 1). Further, it has been revealed that an artificial oligonucleotide obtained by this technique has higher nuclease resistance than a natural oligonucleotide and is less likely to be decomposed in cells.
However, a nucleic acid itself is negatively charged and cannot pass through the cell membrane. Therefore, nucleic acid medicines require a drug delivery system (DDS) that allows nucleotides to pass through the cell membrane. At present, a lipofection technique using liposomes has been developed as a technique for introducing nucleotides into cells. In this technique, a complex is formed by binding a positively-charged cationic liposome around negatively-charged DNA to allow the DNA etc. to be incorporated into cells through cell surfaces by an endocytosis phenomenon.
However, there is a problem that a lipofection reagent used in the lipofection technique is toxic to the liver and the kidney. Further, the lipofection technique is a DDS utilizing mere endocytosis, and therefore lacks in cell selectivity.
In order to solve these problems, a chemically-modified oligonucleotide has been developed which is obtained by introducing asialoglycopeptide chains at the 3′ end of an oligonucleotide so as to be introduced into cells through an asialoglycoprotein receptor (ASGPR) (Patent Literatures 2 and 3). However, as shown in FIG. 4, a chemically-modified oligonucleotide obtained by this method has a complicated chemical structure having three asialoglycopeptide chains, and its synthesis requires complicated operations.